For internal combustion engines, such as diesel engines, nitrogen oxide (NOx) compounds may be emitted in the exhaust. To reduce NOx emissions, a selective catalytic reduction (SCR) process may be implemented to convert the NOx compounds into more neutral compounds, such as diatomic nitrogen or water, with the aid of a catalyst and a reductant. The catalyst may be included in a catalyst chamber of an exhaust system, such as that of a vehicle or power generation unit. A reductant, such as anhydrous ammonia, aqueous ammonia, diesel exhaust fluid (DEF), or aqueous urea, is typically introduced into the exhaust gas flow prior to the catalyst chamber. To introduce the reductant into the exhaust gas flow for the SCR process, an SCR system may dose or otherwise introduce the reductant through a dosing module that vaporizes or sprays the reductant into an exhaust pipe of the exhaust system up-stream of the catalyst chamber. The SCR system may include one or more sensors to monitor conditions within the exhaust system.
Once the reductant is introduced into the exhaust gas flow, the two need to be mixed. WO 2018/226626 A1 discloses a multi-stage mixer which is configured to receive exhaust gas and reductant and mix the reductant with the exhaust gas to provide the exhaust gas mixed with reductant to a catalyst. The application recognizes that it is beneficial to provide the catalyst with a substantially uniform flow of exhaust gases and reductant, facilitate substantially uniform reductant distribution in the exhaust gases downstream the multi-stage mixer and provide a relatively low pressure drop in a relatively compact space compared to conventional aftertreatment systems. The known multi-stage mixer uses a Venturi to introduce a swirl mix into the exhaust flow and uses a radial offset of that Venturi from the center axis of the multi-stage mixer so as to cause any reductant build up on the Venturi body to be substantially redistributed to the exhaust gases downstream of the multi-stage mixer.
However, the known multi-stage mixer, while a significant improvement over the prior art, is not proportionally scalable over the complete range of diesel engine systems due to restrictions on space claim. Thus, so as to allow better scaling while still causing any reductant build up on the Venturi body to be substantially redistributed to the exhaust gases downstream of the mixer, an improved mixer would be desirable.